Occurrence and Ecological Impacts of Microplastics in Soil Systems: A Review

Abstract

Microplastics, as a group of emerging contaminants, are receiving growing attention. During the last decade, their occurrence and toxicity in aquatic ecosystems have been intensively studied and reviewed, but less attention has been paid on soil ecosystems. Given the importance of soil ecosystems and the call for increasing research on soil from scientific communities, it is predicted that relevant studies will boom in the following years. The present review intends to provide a comprehensive overview of current knowledge on microplastic pollution in soil environments. We critically summarize the source, contamination level and fate of microplastics in (industrial and arable) soils. Then, we thoroughly describe what effects have been observed on soil microbes, animals and plants, and analyze what insights we can get from available information. Finally, we identify knowledge gaps that need to be filled and give suggestions for future research.

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Bulletin of Environmental Contamination and Toxicology
https://doi.org/10.1007/s00128-019-02623-z
FOCUSED REVIEW
Occurrence andEcological Impacts ofMicroplastics inSoil Systems:
AReview
FengxiaoZhu1· ChangyinZhu1· ChaoWang1· ChengGu1
Received: 15 January 2019 / Accepted: 22 April 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Microplastics, as a group of emerging contaminants, are receiving growing attention. During the last decade, their occur-
rence and toxicity in aquatic ecosystems have been intensively studied and reviewed, but less attention has been paid on soil
ecosystems. Given the importance of soil ecosystems and the call for increasing research on soil from scientific communities,
it is predicted that relevant studies will boom in the following years. The present review intends to provide a comprehensive
overview of current knowledge on microplastic pollution in soil environments. We critically summarize the source, contami-
nation level and fate of microplastics in (industrial and arable) soils. Then, we thoroughly describe what effects have been
observed on soil microbes, animals and plants, and analyze what insights we can get from available information. Finally, we
identify knowledge gaps that need to be filled and give suggestions for future research.
Keywords Microplastic· Distribution· Impact· Soil· Biota
Introduction
Microplastics are generally defined as plastic parti-
cles < 5mm (Rillig etal. 2017). They have attracted increas-
ing concerns worldwide over the last decade, and extensive
studies have been conducted on their occurrence and impacts
in aquatic environments. Typical microplastics encountered
are shown in Fig.1.
Recent studies based on aquatic species have shown that
microplastics could be ingested/accumulated by aquatic
animals and cause detrimental effects on their survival and
health (Auta etal. 2017; Frydkjær etal. 2017). Given the
central role of soil in maintaining biodiversity, mediat-
ing nutrient cycling and providing food, it is imperative to
figure out how microplastics affect our terrestrial environ-
ments (especially soil environments), which has been largely
neglected during the past years. It was reported that soils are
probably receiving much more plastic wastes than the oceans
(Horton etal. 2017). Therefore, research is greatly needed
to focus on the problem of microplastic pollution in soil.
Indeed some recent studies (mostly published in
2016–2019) have begun to investigate the contamination
level and possible sources of microplastics in soil, as well
as their effects on the fitness of soil organisms (Huerta
Lwanga etal. 2016; Rodriguez-Seijo etal. 2017; Zhang and
Liu 2018). The results from these studies tend to confirm
that microplastics are ubiquitous and persistent contaminants
in soil as they were observed in the ocean (Zhang and Liu
2018), and that microplastics can affect the survival, growth,
reproduction, feeding and immune system of soil organisms
(Huerta Lwanga etal. 2016; Zhu etal. 2018a).
Therefore, the aim of this review is to provide an over-
view of current knowledge on the occurrence and likely eco-
logical impacts of microplastics in soil systems, and then to
outline the possible future research directions.
Microplastic pollution insoil systems
Sources
Microplastics can enter soil environments via multi-
ple routes, which has recently been reviewed by Bläsing
and Amelung (2018). In this paper, we are giving a con-
cise but comprehensive description, with new evidences
incorporated.
* Cheng Gu
chenggu@nju.edu.cn
1 State Key Laboratory ofPollution Control andResource
Reuse, School oftheEnvironment, Nanjing University,
Nanjing210023, China

Bulletin of Environmental Contamination and Toxicology
1 3
(1) Land application of sludge and organic fertilizer may
introduce microplastics into soils. Previous studies
show that up to 90% of the microplastics from the
influent wastewater would be retained and accumu-
lated in the sludge, and the concentrations of micro-
plastics in sludge range from 1500 to 56,400 particles
kg−1 (Li etal. 2018; Mintenig etal. 2017). The pres-
ence of microplastics in organic fertilizers (up to 895
particles kg−1) has also been documented (Weithmann
etal. 2018). Hence, long-term application of sludge
and organic fertilizers may lead to soil pollution with
microplastics, which is evidenced by some previous
studies (Zubris and Richards 2005; Zhang and Liu
2018).
(2) Agricultural plastic film is another source for micro-
plastic pollution in soil. Plastic mulching has become
a widely used agricultural practice in many countries
for its instant economic benefits (Steinmetz etal. 2016).
For example, in 2015, mulch film consumption reached
1.455 million tons in China (Luo etal. 2018). The prob-
lem is that, it is not technically feasible to remove or
recycle most of the mulch films from the field because
they are usually very thin (0.01–0.03mm). Films
remaining in the field can slowly fragment into smaller
particles by a combination of physical, chemical and
biological effects (Barnes etal. 2009; Briassoulis etal.
2015), resulting in microplastic pollution.
(3) Atmospheric deposition may also serve as a significant
source of microplastics entering the surface soil. The
atmospheric fallout of microplastics in the urban areas
of Paris was estimated to be 2–355 particles m−2days−1
(Dris etal. 2016). In addition, detection of microplas-
tics in soils from remote unsettled high mountain areas
(Scheurer and Bigalke 2018), suggests that air deposi-
tion can be the major source in some areas.
(4) Other sources, such as wastewater irrigation, littering,
and surface runoff, may also be contributors to soil
microplastic pollution (Bläsing and Amelung 2018).
Overall, agricultural soils may receive microplastics
mainly from sludge/compost fertilization, plastic mulching
and wastewater irrigation. Whereas, air deposition might be
an important source for forest, urban and industrial soils
where regular fertilization and irrigation is not necessary.
However, since microplastic concentrations in fertilizer/
water/air can be highly variable and source studies are at an
early stage, the exact role of presumably important sources
for soils of different land uses is still unclear.
Fig. 1 Typical microplastics encountered in aquatic (and terrestrial)
environments. Polymer type refers to (Andrady 2011; Avio et al.
2017; Scheurer and Bigalke 2018); polymer structure refers to (Ency-
clopædia Britannica 2019); polymer density refers to (Andrady 2011;
Hidalgo-Ruz etal. 2012); microplastic morphotype refers to (Tanaka
and Takada 2016)

Bulletin of Environmental Contamination and Toxicology
1 3
Distribution
Numerous studies have been taken to investigate the distri-
bution of microplastics in marine environments (Auta etal.
2017), but information on the status of microplastic pol-
lution in soil environments is still quite limited. Here, we
provide a summary on relevant studies published recently.
Available data suggest that some industrial areas may
have been heavily contaminated with microplastics. For
instance, Fuller and Gautam (2016) found that soils near an
industrial area in Australia contained 0.03–6.7% of micro-
plastics (mainly PVC). On the other hand, results from
Scheurer and Bigalke (2018) demonstrated that microplastic
pollution in floodplain soils in Switzerland, although ubiqui-
tous, was less severe ( ≤ 0.00555% and ≤ 593 particles kg–1,
mainly PE).
Soil microplastic pollution in China deserves special
attention, since large amounts of plastics are produced,
consumed and discharged in China every year (Gourmelon
2015). Now, a few reports are available on the occurrence
of microplastic pollution in soils (mainly farmland soils)
in China. Results from these studies can be summarized as
follows: (1) In most soil samples, microplastic contents are
low ( ≤ 320 particles kg−1) (Lv etal. 2018; Liu etal. 2018;
Zhang etal. 2018); however, in soils with a history of sew-
age sludge amendment and wastewater irrigation, the con-
tent can be high ( > 7000 particles kg−1) (Zhang and Liu
2018), being higher than the general contents observed in
subtidal zones of the ocean (15–3320 particles kg−1) (Xu
etal. 2018). (2) Small size microplastics ( < 1mm) and fib-
ers are the most abundant ones (Lv etal. 2019; Zhang and
Liu 2018). (3) The main types of microplastics detected are
PE and PP (Lv etal. 2019; Liu etal. 2018). Here, it should
be mentioned that methods used for microplastic extraction
may influence the types of polymers recovered (see notes of
Table1). (4) Microplastics are present not only in top soils
(0–10cm) but also in deep soils (10–30cm). Top soils may
contain higher or lower concentrations of microplastics than
deep soils, which is dependent on the ease of the plastics to
penetrate into deep soil or to escape due to surface runoff
(Zhang etal. 2018).
Given that high microplastic content has been docu-
mented for some industrial, farmland and even forest soils
(Table1), to avoid soil quality deterioration, it is urgent to
conduct large scale and continuous surveys of microplastic
pollution in soils under different land uses. Information on
hotspot zones, major microplastics presented and associated
sources is essential for risk assessment and pollution con-
trol. In addition, to make different studies more comparable,
standardization of the units of measurement is required (Ng
etal. 2018). In a previous study, weight-based data pres-
entation was recommended for soil and sediment pollution
(Zhang etal. 2019). When assessing the contamination level
varying with time, weight-based microplastic content can
also be useful since introduction of new microplastics and
fragmentation of existing microplastics can be distinguished
in this way.
Degradation andtransport
Under natural conditions, microplastics are degraded due
to UV-radiation, thermal oxidation, physical abrasion and
biodegradation effects; during these processes, microplas-
tics undergo changes in polymer chemical structure, such as
chain cleavage, disproportionation, increase in oxygen-con-
taining functional groups, etc. (Luo etal. 2018). But these
processes are very slow (especially in soil) because (micro)
plastics are recalcitrant in nature. Earlier studies showed that
PP degradation in soil was minimal (0.4%) after one year
(Arkatkar etal. 2009) while no degradation was observed
for PVC and PS buried under soil for over 32years (Otake
etal. 1995).
Although optimal conditions may not be met in real envi-
ronments, biodegradation is still one of the most promising
ways to reduce microplastic pollution in the environment
(Auta etal. 2017). Some efforts have been made by exploit-
ing the potentials of terrestrial organisms. Notably, wax-
worms and mealworms are reported to be able to efficiently
digest PE or PS plastics (Brandon etal. 2018; Yang etal.
2015a). Moreover, a range of bacterial and fungal strains
capable of degrading (micro) plastics have been isolated
from the environment or animal guts (Ali etal. 2014; Krue-
ger etal. 2015; Yang etal. 2015b).
Like other pollutants, microplastics in soil can move.
They can travel short distances through bioturbation and
agricultural practices (such as ploughing). Bioturbation-
related microplastic movement receives more interests,
and some earthworm (Rillig etal. 2017) and collembolan
(Maaß etal. 2017) species are found to transport micro-
plastic particles from surface soil to deep soil. In addi-
tion, there are also evidences showing that microplastics
can travel long distances through surface runoff and soil
erosion, by which they can enter water bodies and even
the ocean (Nizzetto etal. 2016). Furthermore, the co-
transport of organic/inorganic pollutants and microplas-
tics (which act as an active adsorbent) may have essential
environmental consequences, which has drawn consider-
able attention in both aquatic and terrestrial ecosystems
(Browne etal. 2013; Wijesekara etal. 2018; Yang etal.
2019).

Bulletin of Environmental Contamination and Toxicology
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Table 1 Available data on the status of microplastic pollution in soil
Notes: (1) /—relevant information is not available. (2) shallow—shallow soil (0–3 cm); deep1—deep soil (3–6cm); top—top soil (0–10cm); deep—deep soil (10–30 cm). (3) a—data are the total
number of total mesoplastic and microplastic particles detected but microplastics accounts for more than 95%; b—The methods used for microplastic extraction in that study may significantly
underestimate the abundance of dense polymers such as PVC and PET. (4) GC–MS stands for gas chromatography-mass spectrometer; FT-IR stands for Fourier transform-infrared spectroscopy
Country Soil source Microplastics Methods for microplastic extraction,
identification and quantification
References
Concentration Major size
(mm)
Major type Morphotype
(%) (particles kg−1)
Australia Near the industrial area 0.03–6.7 / / PVC / Pressurized fluid extraction → GC–MS
and FT-IR spectrophotometer analysis
Fuller and Gautam (2016)
Switzerland Floodplain soils ≤ 0.0055 ≤ 593 < 0.5 PE / Density separation using 27% NaCl
solution → 65% HNO3 treatment of
organic matter → FT-IR microscope
Scheurer and Bigalke (2018) b
China (Shanghai) Rice-fish co-culture ecosys-
tems
/10.3 ± 2.2 < 1 PE, PP Mainly fibers Density separation using saturated NaCl
solutions → 30% H2O2 treatment of
organic matter → Identification under
the microscope
Lv etal. (2019) b
China (Shanghai) Vegetable fields / 78.0 ± 12.9 shallow
62.5 ± 13.0 deep1
< 1 PE, PP Fibers and fragments Density separation with saturated NaCl
solution → 30% H2O2 treatment of
organic matter → µ-FT-IR assay
Liu etal. (2018) b
China (Northwest
area)
Agricultural field ≤ 0.000054 40 ± 126 top
100 ± 141 deep
> 0.1 Low density micro-
plastics (such as
PE and PP) were
targeted
/ Water floatation method → heat treat-
ment of microplastics at 130°C for
3–5s → Identification under the
microscope before and after heat
treatment
Zhang etal. (2018) b
Greenhouse field 100 ± 254 top
80 ± 193 deep
Fruit field 320 ± 329 top
120 ± 129 deep
China (Southwest
area)
Greenhouse vegetable soils / 7100–42,960a < 1 / Mainly fibers Density separation using saturated NaI
solution → H2O2 treatment of organic
matter → method)
Zhang and Liu (2018)
Forest buffer zone / 8180–18,100a < 1 / Mainly fibers

Bulletin of Environmental Contamination and Toxicology
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Ecological impacts ofmicroplastics onsoil
biota
How domicroplastics aect soil microorganisms?
The interaction of microplastics with soil microbiota
remains largely unexplored. Only a few studies have inves-
tigated the effects of microplastics in soil systems, mainly
on overall microbial activity, bacterial transport, and spread
of antibiotic resistant genes (ARGs).
PP particles (7% and 28%) were reported to have a posi-
tive effect on soil microbial activity (Liu etal. 2017), while
polyacrylic (0.05–0.4%), polyester (0.05–0.4%) and PS par-
ticles (1mgkg−1) showed a negative effect (Awet etal. 2018;
de Souza Machado etal. 2018). Since polymer type, shape,
size and concentration varied in these studies, it is difficult
to draw a general conclusion on the toxicity of microplastics
based on their features. Modified soil structure and microbial
community composition have been proposed to be the pos-
sible reasons for altered microbial activity in these studies,
however no direct evidences/linkages have been provided or
observed. Further investigations are needed to improve our
understanding of the effects and mechanisms of microplas-
tics on soil microbial metabolism and activity.
The effect of microplastics on the transport and deposi-
tion of soil microorganisms has not been intensely exam-
ined, but some insights may be gained from the study by He
etal. (2018). The authors found that under low ionic strength
conditions PS particles had negligible effect on Escherichia
coli transport in quartz sand, whereas under high ionic
strength conditions, plastic particles stimulated bacterial
transport. They proposed that the adsorption of plastic par-
ticles onto cell surfaces and the repel effect were the main
driver for the increased cell transport induced by plastics
at nanoscale (20nm), while plastics at microscale (2μm)
mainly increased cell transport by competing for deposition
sites on sand. Further research is needed to investigate how
microplastics affect microbial movement in real soil systems.
Spread of ARGs is an increasing concern, due to its
potential adverse effects on human health. Studies based
on aquatic ecosystems reveal that microplastics can serve
as hotspots of gene exchange between phylogenetically dif-
ferent microorganisms by introducing additional surface,
thus having a potential to increase the spread of ARGs and
antibiotic resistant pathogens in water and sediments (Arias-
Andres etal. 2018; Huang etal. 2019; Imran etal. 2019). In
soil ecosystems, the presence of PS microplastics (0.1%) has
been shown to increase the retention time of antibiotics and
ARGs (Sun etal. 2018). More evidences are needed to draw
a conclusion on whether microplastic pollution facilitates
the transmission of ARGs in soil environments.
How domicroplastics aect soil animals?
Knowledge about the impacts of microplastics on the health
of soil animals lags far behind that of aquatic animals. Only
a few soil invertebrates have been examined, including
nematodes, oligochaeta (e.g. earthworms), collembolan
and isopods. Microplastics were either added in liquid
medium, food or soil matrix in previous studies, to study
their effect on the survival, growth, reproduction, inflamma-
tory response, metabolic activity, feeding behavior, neurode-
generation and gut microbiota of soil animals.
When assessing the toxicological effect of microplas-
tics on nematodes, size is an important factor to be con-
sidered (Lei etal. 2018; Kim etal. 2019). Lei etal. (2018)
chronically exposed Caenorhabditis elegans to 1mg L−1
PS particles (0.1, 0.5, 1.0, 2.0 and 5.0µm) for 3days. They
found that the 1.0μm group had the lowest survival rate, the
shortest average lifespan and the largest decrease in body
length; 1.0μm particles also significantly downregulated the
expression of unc-17 and unc-47 genes, reflecting damages
to cholinergic and GABAergic neurons in nematodes. The
strongest toxicity of 1.0μm PS particles might be due to
that the moderate-sized plastic particles were more readily
taken by nematodes; this hypothesis was supported by the
observation that 1.0μm particles showed higher accumula-
tion than others.
Studies on oligochaeta show that the effect of micro-
plastics is highly dependent on the level of exposure. For
instance, Zhu et al. (2018a) reported a concentration-
dependent effect of PS nanoplastics on the weight of the
soil oligochaete Enchytraeus crypticus: 0.025% (in oatmeal)
having a slightly positive effect; 0.5% having no effect; 10%
having a significantly negative effect; in addition, a clear
shift in the gut microbiota was only observed under the
highest exposure (10%). These findings are in line with the
results from Huerta Lwanga etal. (2016) that 7% PE micro-
plastics in plant litter (corresponding to 0.2% in soil) had no
effect on the growth and survival of the earthworm Lumbri-
cus terrestris but 28–60% addition had an inhibitory effect.
Previous studies also suggest that histological analysis
may be used for early diagnoses when assessing the tox-
icity of microplastics on oligochaeta in soil, and that bio-
degradable plastics are not intrinsically less toxic than
conventional plastics. For instance, Rodriguez-Seijo etal.
(2017) reported that, although addition of low density PE
microplastics (0.0625–1% in soil) showed no effect on the
survival and growth of the earthworm Eisenia Andrei, tissue
damage and immune system responses were observed even
under the lowest exposure level. Qi etal. (2018) reported
that, when applied at the same dosage (1% in soil), micro-
plastics derived from starch-based biodegradable films had
more effects on earthworm growth than conventional low

Bulletin of Environmental Contamination and Toxicology
1 3
density PE films. This was possibly because the biodegrad-
able plastics were mainly composed of PET and polybutyl-
ene terephthalate, which might be more toxic than PE.
Soil collembolan species seem to be sensitive to micro-
plastic pollution. Zhu etal. (2018b) reported that exposure to
0.1% PVC microplastics for 56days, significantly inhibited
the growth (by 16.8%) and reproduction (by 28.8%) of Folso-
mia candida in soil, and significantly modified the metabolic
turnover of this animal (as indicated by changes in δ15N and
δ13C values). Recently, Ju etal. (2019) reported a similar neg-
ative effect of PE microplastic exposure (0.1–1%) on Folso-
mia reproduction. In both studies, altered animal gut bacterial
community due to microplastic exposure were also observed.
These results suggest that collembolan may be used as a valu-
able bioindicator of microplastic disturbance in soil.
Isopods are commonly used as test species in ecotoxicity
studies, due to their important role in plant litter decomposi-
tion processes (Drobne 1997) Kokalj etal. (2018). assessed
the effects of PE microplastics presented in food pellets
(0.4%) on the feeding behavior and energy reserve of iso-
pods. After 14days exposure, no effects on any end-point
(including food ingestion rate, defecation rate, food assimi-
lation rate and efficiency, body mass change, mortality and
energy reserves in the digestive glands) were observed, sug-
gesting little hazardous effects of PE microplastics to the iso-
pod Porcellio scaber. Further work is needed to investigate
the potential longer-term effects of such exposure, as well as
the effects of other commonly detected microplastics in soil.
How domicroplastics aect plants?
When it comes to plants, people are concerned about two
questions: whether plant can absorb and accumulate micro-
plastics, and how microplastics affect plant growth and
food quality. Currently, such information is scarce, possibly
because it is difficult to identify microplastics in plant tissues
and the effect on crops has not attracted enough attention.
It is likely that small-sized microplastics can overcome
cell wall and membrane barriers. The possibility of plant
uptake of microplastics can be investigated with the aid of
fluorescent microbeads. For example, a cell culture-based
study demonstrated that nano-scale ( < 100nm) fluorescent
PS beads could enter tobacco cells through endocytosis
(Bandmann etal. 2012). More importantly, a recent study
based on whole plant cultures showed that edible plant could
uptake and accumulate micro-scale (0.2μm) fluorescent PS
beads from soil (Li etal. 2019), highlighting the potential
risks of microplastic uptake by humans via food web chain.
Currently, only one study has been carried out to inves-
tigate the impacts of microplastics on plants. By adding
1% biodegradable and PE plastic particles in soil, Qi etal.
(2018) found that both types of microplastics disturbed the
growth of wheat, with the former having a stronger negative
effects than the latter. Fruit biomass was also negatively
affected by biodegradable plastic particles. Interestingly, the
presence of earthworm alleviated the impairments in wheat
induced by microplastics. In this study, the accumulation of
PE particles in plant tissue was not examined.
The role ofmicroplastic‑associated organic
orinorganic pollutants inmicroplasticinduced
stresses
It is noted that in previous studies, microplastics are often
considered as pure polymers or pure physical particles.
In fact, microplastics may contain substantial amounts of
chemical additives added intentionally (such as plasticiz-
ers and flame retardants) or toxic pollutants adsorbed from
the surrounding environment (such as polycyclic aromatic
hydrocarbons and heavy metals) (Hong etal. 2017), which
could be a real hazard to soil organisms. At present, the
role of these organic or inorganic pollutants in microplas-
tic-induced stresses has drawn little attention, although the
possibility of pollutant transfer from soil microplastics to
earthworms has been demonstrated (Gaylor etal. 2013). In
other ecosystems, microplastic-associated pollutants have
been shown to play a vital role in determining the toxicity of
microplastics to marine animals (Browne etal. 2013; Olivi-
ero etal. 2019) or sludge digestive microbiota (Wei etal.
2019). It suggests that pollutants associated with environ-
mental microplastics should be considered in further toxi-
cological studies. Particular attention should be paid to the
pollutants that are highly toxic or at high concentrations, as
not all pollutants are sufficient to cause a significant negative
effect (Zhang etal. 2019).
Knowledge gap andfuture recommendations
Based on this review, we can see that although our under-
standing of microplastics in soil environments is advancing,
there is still a remarkable lack of relevant data. For example,
the characteristics of microplastic pollution in soil environ-
ments, their potential ecological effects and the underpin-
ning mechanisms of their toxicity are far from fully under-
stood. Therefore, in future studies, the most important issues
needed to be addressed are as follows:
(1) Firstly, we need to understand the distribution of
microplastics in soil ecosystems, and answer basic
questions like: What is the extent of microplastic
pollution in soils of different land uses? What are
the major sources? Which microplastics (in terms of
polymer type, shape and size) are the most abundant
ones? Although current literatures suggest PE and PP
polymers, small size ( < 1mm) particles, and fibers are
generally more abundant than their counterparts, more

Bulletin of Environmental Contamination and Toxicology
1 3
evidences are needed to confirm whether it is true since
methods used in most previous studies have underesti-
mated the abundance of dense particles (such as PVC
and PET). In addition, considering that additives and
environmental contaminants associated with micro-
plastics may have a profound effect on the toxicity of
microplastics, it is better to include information on the
concentration of these compounds in future surveys.
(2) Then, we need to understand their ecological effects
and associated controlling factors, so that we can
answer critical questions like: How do microplastics
affect the mobility, abundance, diversity, composition
and function of soil organisms? Would an effect be
observed at environmentally relevant concentrations?
How do microplastic features and soil type influence
the ecological effects of microplastics? This informa-
tion is the basis for a precise risk assessment.
(3) Meanwhile, we need to get a better understanding about
the mechanisms of the ecological effects observed. For
instance, are microplastics mainly acting as a physical
or chemical hazard? Currently little has been done to
clarify the contribution of chemical (degradation prod-
ucts, polymer additives or environmental chemicals
adsorbed on the surface) release from microplastics
to their toxicity in the context of soil; the molecular
mechanisms of microplastic-induced ecotoxicity are
also unclear. We believe that they are among the most
important questions remaining to be answered in this
area.
Acknowledgements This work was financially supported by National
Key Research and Development Plans (2018YFC1800602) and the
National Science Foundation of China (21777066).
Compliance with Ethical Standards
Conflict of interest The authors declare that there is no conflict of in-
terest.
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